This micrograph shows the grain structure of a roll-bonded aluminium alloy diffusion couple in the as-rolled condition. The upper half of the couple is Al-0.5Fe-1.0Mn (wt.%), the lower half is Al-1.0Si (wt.%). Rolling heavily deforms the microstructure, producing thin, elongated grains parallel to the roll-bonded interface.The Barker's etch produces a thick oxide layer on the grains of aluminium (anodising). When viewed in cross-polarised light, interference in the oxide layer produces colours which depend on grain orientation, hence the grain structure is imaged. Prior to roll-bonding the Al-Si alloy was solution-treated, creating recrystallized grains that were much larger than the cellular microstructure of the Al-Fe-Mn alloy. This difference in scale of microstructure can be seen in the rolled diffusion couple.